As is known, heating system boilers firing solid fuels as wood, coal, and residual products are gaining a widespread acceptance and among these, one boiler wherein combustion takes place by natural draft and with a downwardly directed flame from a fuel supporting grate.
This last boiler type, while offering some definite advantages, also has some disadvantageous features, which are shared with some other boiler types, such as the impossibility of controlling, but in a coarse way, the combustion process with consequent poor performance, some starting difficulties, and the need for large section area chimneys.